Development of sweating ability in winter- and summer-born Friesian calves aged 1 to 6 weeks.

Sweating rate, rectal and skin temperatures and respiration rate were measured at weekly intervals from 7 days of age (for 4 weeks in Experiment 1; 6 weeks in Experiment 2) in winter- and summer-born Friesian calves exposed to a temperature of 39 degrees C dry bulb and 32 degrees C wet bulb in a climate chamber. Four calves were studied in each season in both experiments. In Experiment 1, ambient temperatures were from 3 degrees to 9 degrees C higher in early summer than in late winter. During each 39 degrees C exposure, sweating rate increased from basal levels of 40-90 to plateau levels of 120-300 g/m2 per h after 90-120 min. The increase in sweating rate with age was most pronounced in winter-born calves, but summer-born calves had higher values at 1 week of age (167 +/- 52.4 vs 94.4 +/- 30.1 g/m2 per h). Seasonal differences in ambient temperature were greater in Experiment 2 (11 degrees to 17 degrees C). In this case summer-born calves had higher sweating rates at each age (plateau values of 220-320 g/m2 per h), and showed a more rapid increase in sweating rate during each 39 degrees C exposure than winter-born calves (plateau values of 100-250 g/m2 per h). The results demonstrate major changes in sweating competence during the first 4-6 weeks of life in Friesian calves, a quite pronounced effect of season (ambient temperature) on the levels of sweating achieved, and indicate that low sweating rates in newborn calves are a contributing factor in deaths due to hyperthermia in semi-arid grazing areas.